1. (Field of the Invention)
The present invention relates to a galvannealed steel sheet suitable for use in automobiles as high strength steel sheet, plated with a coating having excellent powdering resistance, low temperature chipping resistance, and paint bake hardenability,as well as to a manufacturing method thereof.
2. (Description of the Related Art)
The galvannealed steel sheet has been in extensive use in recent years as steel sheet suitable for manufacturing automobile bodies. The galvannealed steel sheet is manufactured by applying a Fe--Zn alloying process to a hot-dip galvanized steel sheet, wherein the hot-dip galvanized steel sheet is heated up to a temperatures in the range from 500 to 600.degree. C. for a retention time of 3 to 60 minutes. By means of Fe--Zn alloying, a Zn layer is turned into a Fe--Zn alloy layer containing 8 to 15 wt. % of Fe normally. In the case of a plated coating being composed of the Fe--Zn alloy layer, coating adhesion between the plated coating and paint is superior to an ordinary hot-dip galvanized steel sheet. Spot weldability is also improved in addition to markedly enhanced corrosion resistance after painting. Powdering resistance is degraded if the amount of Zn coating on the galvannealed steel sheet is excessive. Accordingly, the amount of the Zn coating is normally 20 to 70 g/m.sup.2 of the surface of the steel sheet on one side.
In application of the galvannealed steel sheet described above for manufacturing automobile bodies, the following properties are required of the steel sheet with respect to formability thereof and corrosion resistance of a galvannealed coating:
(a) exfoliation of the galvannealed coating does not occur during plastic working of the steel sheet (powdering resistance), PA1 (b) excellent adhesion property of the galvannealed coating is maintained even in low temperature environments (low temperature chipping resistance), PA1 (c) a base metal has sufficient strength to be able to cope with requirement for higher strength, maintaining good formability, and PA1 (d) the steel sheet has adequate paint bake hardenability. PA1 C: 0.004-0.008%; Si: 2.5.times.P(%)-0.20%; PA1 Mn: 0.10-0.40%; P: 0.017-0.045%; PA1 sol. Al: 0.003-0.08%; S: 0.015% or less; PA1 Ti: 0.002-0.015%; Nb: 0.010-0.030; PA1 Ti (%)+Nb(%): 0.012-0.035%; N: 0.004% or less; PA1 balance: Fe and inevitable impurities.
Powdering is a phenomenon wherein the plated coating is broken into fine pieces and exfoliated in a region where the steel sheet is subjected to compressive deformation during press working, generating powdery broken pieces of the plated coating. Fe--Zn alloy, being harder than ordinary Zn metal, the galvannealed coating is susceptible to pulverization when subjected to compressive deformation. Not only corrosion resistance is degraded at sites where powdering occurs, but also powdery pieces of the exfoliated galvannealed coating, adhered to a press die, give rise to a cause for defects on the surface of a formed product.
Low temperature chipping is a phenomenon wherein the galvannealed coating exfoliates from the interface between a base metal and the coating when deformation due to impact is caused to occur to a painted galvannealed steel sheet in low temperature environments. The low temperature chipping some times occurs when gravel, and the like collides with the painted surface of the body of an automobile being driven in a cold region, and the like, and it is highly desired to overcome this problem.
Enhanced strength is called for in order to reduce the weight of an automobile body from the viewpoint of improving fuel efficiency, and also to enhance dent resistance of exterior components of an automobile, such as door, and the like. The steel sheet for use in automobile bodies, having excellent formability, normally calls for tensile strength on the order of 300 MPa. However, the steel sheet having tensile strength in the range of 340 to around 400 MPa is in great demand nowadays to lighten the weight of an automobile body.
Paint bake hardenability (hereinafter referred to simply as "BH property") is a property of a metal for age hardening in a range of temperatures for baking paint although unsusceptible to age hardening at room temperature (a steel sheet having BH property is referred to hereinafter simply as "BH type steel sheet"). When press forming is performed on a BH type steel sheet, it maintains a yield strength as low as that of a steel sheet at the time of production thereof Accordingly, accurate shapes of automobile components are obtainable by press forming of the BH type steel sheet. The yield strength of the BH steel sheet goes up due to the effect of age hardening by paint baking after press forming thereof, thus increasing strength of a product after painting.
In other words, by press working of the BH type steel sheet, accurate shapes of components are obtainable with ease, and after painting, high strength of components is obtained. These are highly necessary properties of steel sheets used for manufacturing exterior components of an automobile, requiring both accurate shapes and dent-defect- resistance property (hereinafter referred to as dent resistance).
It means that the galvannealed steel sheet used for an automobile body calls for not only press formability, high strength, and BH property but also excellent adhesion property of the galvannealed coating, that is, powdering resistance, and low temperature chipping resistance.
Various measures for improving powdering resistance are proposed including optimization of an amount of Zn coating, an amount of Al added to a plating bath, and Fe content of the galvannealed coating, and further, restriction on such amounts of alloying elements added to the base metal as may adversely affect the adhesive property of the plated coating.
Both powdering and chipping are phenomena wherein the plated coating exfoliates. Therefore, it was believed in the past that improvement in powdering resistance would be accompanied by improvement in low temperature chipping resistance. In fact, however, improved powdering resistance does not necessarily result in improvement in low temperature chipping resistance. It has since been found that low temperature chipping resistance is primarily dependent on adhesion property at the interface between the plated coating and the surface of the base metal. Accordingly, a method is proposed whereby adhesion property of the plated coating on the surface of the base metal is enhanced.
A galvannealed steel sheet having excellent formability and a manufacturing method thereof are disclosed in Japanese Patent Laid-open (Kokai) No. 2-97653 (1990). The steel sheet described above is characterized in that when same is cut after painted, a plated coating is not susceptible to exfoliation readily from the surface of a cut end thereof. It is said that the steel sheet has a structure formed by intrusion and diffusion of Zn into grain boundaries of the surface of a base metal thereof with Zn each substantially in the shape of a wedge residing on the grain boundaries, preventing the plated coating from exfoliating from the surface of the base metal easily.
The steel sheet as described above can be manufactured by plating the base metal in a plating bath containing Al in a concentration higher than that for normal cases, and by applying the Fe--Zn alloying process thereto at higher temperatures than in normal cases for an extended period of time. However, the Fe--Zn alloying process applied at high temperatures tends to impair powdering resistance, and a longer process time of the Fe--Zn alloying lowers productivity. Furthermore, such a method of processing as described above is regarded effective in prevention of exfoliation of the plated coating from the surface of the cut end of the steel sheet, however, its relationship with chemical composition of the base metal, and its effect on low-temperature chipping resistance are yet to be made clear.
Elements such as Si and P have an effect of increasing strength of the steel sheet without sacrificing much formability thereof, and besides, are inexpensive alloying elements. Therefore, Si and P are alloying elements suited for increasing the strength of the steel sheet. However, Si and P hinder amenability of the base metal to plating. It is known that addition of excessive Si to the base metal results in non-plating, and slowing-down of the alloying velocity. It is also known that P is an alloying element responsible for slowing down the alloying velocity. This is believed to be due to a tendency of P being segregated around the grain boundaries of the base metal, restraining reaction between the grain boundaries of the base metal containing P in high concentration and Zn. Accordingly, it is not easy to achieve both higher strength of the steel sheet and amenability thereof to plating by use of these elements.
A galvannealed steel sheet having excellent adhesion between a plated coating and a base metal, suitable for use as painted steel sheet for application to home appliances, and as steel sheet for automobiles, is disclosed in Japanese Patent Laid-open (Kokai) No. 6-81099 (1994). According to the aforesaid invention, an ultra-low carbon steel containing Si with decreased amount of P is used as the base metal for improving adhesion property thereof, and the surface of the steel, at the interface between the plated coating and the base metal after the Fe--Zn alloying process, is rendered rough with intense asperities so that coating adhesion is improved, enhancing chipping resistance after painting. P content in the base metal is controlled at a level as low as possible, and C content, regarded detrimental for improving coating adhesion, is also held at an ultra-low level, being kept stable with addition of Ti.
It is reported (by W. van. Koesveld et al: GALVATEC '95 Conference Proceedings, p. 343-353) that intrusion of Zn into the grain boundaries of a base metal is promoted by addition of Si to an ultra-low carbon steel with Ti added thereto, enhancing coating adhesion at the interface between a plated coating and said steel as the base metal. However, the technology disclosed in the above report is intended for application to IF steel of low strength, of which the base metal contains no solute C, and no mention is made to P-bearing steel of high strength.
A galvannealed steel sheet with BH property, having excellent powdering resistance, and a manufacturing method thereof are disclosed in Japanese Patent Laid-open (Kokai) No. 4-80349 (1992), for example. According to the aforesaid invention, P content of the steel sheet is controlled at 0.03 percent or lower for enhancing adhesion property of a plated coating while B is added for holding powdering in check. Further, the steel sheet contains Nb 2 to 7.5 times as much as C content therein for obtaining BH property. According to description of the invention, powdering resistance is improved, however, no mention is made of low temperature chipping resistance. In the case of the steel sheet, P content held down at a low level and there is also a risk of its formability being impaired with addition of B.
A cold rolled steel sheet and a hot-dipped galvanized steel sheet, having excellent BH property and secondary workability, and a manufacturing method thereof are disclosed in Japanese Patent Laid-open (Kokai) No. 5-195148 (1993). In the case of the steel sheets according to the invention, an ultra low carbon steel containing Si and P is used as the base metal. Its BH property and secondary workability are said to be obtained by keeping a ratio of Ti content to S content in an optimum range so that species of precipitates are controlled. However, neither Si nor P is added to the galvanized steel sheet described in an embodiment of the invention. Further, neither any evaluation is made on adhesion property of a plated coating nor any mention is made on low temperature chipping resistance.
Thus, a number of proposals have been made for various steel sheets excelling in respective properties, for example, a galvannealed steel sheet having BH property, same having excellent powdering resistance, or same having excellent low temperature chipping resistance, and the like. However, a steel sheet having all the properties including strength, BH property, powdering resistance, and low temperature chipping resistance, all of which are above predetermined levels, has not been developed as yet.